The previous scheme set the reference to main channel to nil and then
drained it. That was racy and hence could lead to deadlock in the
following scenario:
1. multiple senders obtain the main channel from the stored reference
2. main channel reference is set to nil
3. main channel is drained
4. multiple senders send to main channel
So the issue here is that the entire forwarder stopping sequence
occurs between senders obtaining of the reference to the main channel,
and then sending on that channel. If more senders sent than there is
capacity in the channel then this leads to deadlock.